Nearly all of today's electronic devices rely on semiconductor components known as integrated circuits (ICs), which are most commonly manufactured through a process known as optical lithography. Lithography is a patterning method that uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical “photoresist”, or simply “resist,” on to a substrate, such as a wafer of semiconductive material. A series of chemical treatments are used to either engrave the exposure pattern into the substrate, or enable new material to be deposited in the desired pattern to be deposited onto the substrate. By repeating this process using different photomasks, layers of an integrated circuit are gradually formed on the substrate. In complex integrated circuits, a modern CMOS wafer may go through the photolithographic cycle up to 50 times. The state of the art in optical lithography, particularly the wavelength of the light being used, is a gating factor in the ability to make increasingly smaller features on ICs.
Generally speaking, optical lithography begins with cleaning step, where a wet chemical treatment is used to remove any contaminants from its surface. The wafer may then be dried by heating it to a temperature sufficient to drive off any moisture present on the wafer surface. A liquid or gaseous “adhesion promoter”, such as Bis(trimethylsilyl)amine (“hexamethyldisilazane”, HMDS), may then be applied to the wafer surface in order to promote adhesion of the photoresist.
Next, the wafer is coated with a layer of photoresist by spin coating. Photoresist describes a category of light-sensitive materials. In the case of a so-called “positive photoresist,” the exposure to light causes a chemical reaction in the photoresist, which makes the exposed photoresist soluble in a “developer” solution. (With a “negative photoresist,” unexposed regions are soluble in the developer.) Typically, a liquid solution of photoresist is dispensed onto the wafer, and the wafer is spun rapidly to produce a uniformly thick layer. The wafer is then exposed to a pattern of intense light via a photomask, causing the exposed photoresist to become soluble. The exposed photoresist is then removed, exposing regions of the substrate corresponding to the pattern created by the photomask.
Photomasks may be protected from particle contamination, a significant problem in semiconductor manufacturing, by a thin transparent film called a pellicle, mounted to one side of the photomask. The pellicle is placed far enough away from the mask so that particles that land on the pellicle will be out of focus and therefore not transferred to the wafer. Consequently, the optical properties of the pellicle must be taken into account.
After the soluble photoresist is removed, a chemical etching agent may applied to remove the uppermost layer of the substrate in any areas that are not protected by photoresist. Alternatively or additionally, material may be deposited into areas not protected by the substrate. The remaining photoresist is removed and the substrate now with a pattern corresponding to the photomask on its surface. This process may then be repeated using a different photomask for each layer of the desired integrated circuit.
With the progress of technology and the reduction of the feature size, the wavelength of the exposure light had to be reduced several times. Currently, the 193 nm lithography combined with immersion and double patterning technology is the state of the art.
Shorter wavelength lithography, known as next generation lithography (NGL), has been studied in order to produce IC with even smaller features. NGL uses shorter ultraviolet light (157 nm), extreme ultraviolet (EUV) light (e.g. 13.5 nm), X-ray (0.4 nm), and the even shorter wavelengths of electron and ion beams. Due to its optical characteristics, EUV lithography is generally accepted as the natural extension of optical lithography and is currently the most promising NGL technology. However, to this day, research and development of EUV technology has cost several billion US dollars worldwide. A single EUV exposure tool costs about US$70 million.
While most other NGLs require one-fold image reduction membrane masks, EUVL uses masks with four-fold image reduction, which makes mask fabrication feasible with current technology. However, in abandoning 157 nm lithography, the industry has created a technological jump from 193 nm to 13.5 nm wavelength, creating complex challenges across the board. Therefore, EUVL technology includes EUV resist technology, EUV aligners or printers, and EUV masks, as well as metrology, inspection, and defectivity controls.
One important aspect to bear in mind is the fact that all available materials are strong absorbers of EUV light and no material is transparent enough to make use of refractive optics (e.g. lenses). Therefore, it is necessary to make use of reflective optics only (e.g. mirrors) in EUVL optical systems.
However, challenges are present in almost every aspect of EUVL technology. Some challenges are common to all NGL technologies, e.g. resist resolution and line-edge roughness (LER). Other challenges are unique to EUVL, e.g. resist outgassing owing to the EUVL high-vacuum environment. In the past 20 years the main topics of research in EUVL have been: source, optics, mask, multilayer coating, resist, metrology, reticle handling, defects, and contamination control. It is, for example, a critical task to create a defect-free EUVL mask. EUVL mask technology includes mask blank preparation and pattern fabrication. Additionally, because of the harsh environment necessary for EUV lithography, it has previously been difficult or impossible to produce suitable protective pellicles, strong enough to shield the photomask from debris while still being suitably transparent to EUV light. Thus, what is needed is an EUV transparent membrane that is resilient enough to withstand the harsh vacuum environment necessary for EUV lithography.